With the emerging technologies of cellular networks, embedded systems, and the Internet, mobile communication devices such as a smartphone or a tablet have become popular because they combine the mobility of a cellular phone with the functionality of a computer or a personal digital assistant (PDA) into a single device. As such, many of these devices are equipped with a global navigation satellite system (GNSS) module that utilizes a network of orbiting satellites for position determination and/or navigation functions. However, the recent proliferation of wireless local area networks (WLANs) has made it possible for mobile devices to use Wi-Fi access points associated with one or more WLANs as an alternative method for position determination, especially in areas where there is a large concentration of Wi-Fi access points (e.g., urban cores, shopping malls, office buildings, and so on). Moreover, WLAN navigation systems can be advantageous over satellite navigation systems because of satellite signal coverage limitations. For example, while GPS or GNSS signals may not be readily available inside a shopping mall, wireless signals generated by Wi-Fi access points inside the shopping mall are typically detectable by a Wi-Fi enabled mobile communication device.
For WLAN navigation systems, the locations of the Wi-Fi access points are used as reference points from which well-known trilateration techniques can determine the location of a Wi-Fi enabled mobile device. More specifically, the mobile device can use the received signal strength indicators (RSSI) associated with a number of visible access points as indications of the distances between the mobile device and each of the detected access points, where a stronger RSSI means that the mobile device is closer to the access point and a weaker RSSI means that the mobile device is further from the access point. The mobile device can also use the round trip time (RTT) of signals transmitted to and from the access points to estimate the distances between the mobile device and the access points, where the RTT indicates the signal propagation times of a message sent from the mobile device to the access point and a corresponding acknowledgement message sent from the access point to the mobile device. Once these distances are estimated, the location of the mobile device relative to the access points can be determined using trilateration techniques.
Although RTT ranging techniques typically provide more accurate distance measurements than RSSI techniques, RTT ranging techniques typically consume more power than RSSI ranging techniques. More specifically, while the RSSI ranging technique is a passive process in which the mobile device listens for beacon signals broadcast by the access points and then correlates the beacon signal strength to a distance, the RTT ranging technique is an active process in which the mobile device transmits a data packet to the access point, listens for a reply packet transmitted back from the access point, calculates the total travel time of the data and reply packets, and then correlates the packet travel time to a distance. Similarly, maintaining a mobile device's GNSS module in an operational state for position determination and/or movement detection is an active process that consumes an undesirable amount of power (e.g., due to the continuous monitoring of GPS signals and performing calculations associated with triangulation operations).
Because the power supply in mobile devices is often limited to batteries, any unnecessary power consumption is undesirable. Moreover, unnecessary power consumption associated with active processes such as RTT ranging operations and scanning for nearby access points is exacerbated when the mobile device is not in proximity with a WLAN. Similarly, unnecessary power consumption associated with active processes such as scanning for nearby Bluetooth devices and scanning for nearby cellular base stations is exacerbated when the mobile device is not in proximity with any Bluetooth device or cellular base stations, respectively.
Accordingly, there is a need to reduce the power consumption of mobile devices without sacrificing the ability to determine location and/or to scan for nearby devices.
Like reference numerals refer to corresponding parts throughout the drawing figures.